1. Field of the Invention
The present invention relates to a system for controlling a lock-up clutch, which is arranged in parallel with a hydraulic power transmission such as a torque converter, and, more particularly, to a control system for executing a slip control to keep the lock-up clutch in a slip state.
2. Related Art
As well known in the art, the hydraulic power transmission such as the torque converter is inevitably influenced by a relative rotation between its input element and output element so that it causes deterioration in the fuel economy of the vehicle. In recent years, therefore, a lock-up clutch is arranged in parallel with the hydraulic power transmission to connect the input element and the output element directly so that the fuel economy may be improved by applying the lock-up clutch selectively to transmit the torque through the lock-up clutch in place of the hydraulic power transmission.
In short, the lock-up clutch effects a mechanically direct connection between a prime mover such as an engine and a speed change unit. Thus, the lock-up clutch transmits the fluctuation in the output torque of the prime mover as it is to the speed change unit, and may deteriorate the shocks and the booming noises. Considering the driving comfort of the vehicle, therefore, the region where the lock-up clutch can be applied is restricted. If the control is made to apply or release the lock-up clutch completely, on the other hand, the so-called "slip" in the hydraulic power transmission disappears or appears to change the output shaft torque of the vehicle. This torque change is physically felt, if extreme, as the apply shocks to degrade the driving comfort of the vehicle.
In the prior art, therefore, there has been developed a technique, in which the lock-up clutch is not only controlled to apply and release states but also set to have a slightly smaller transmission torque capacity than the input torque so that it may be brought into a slip state (i.e., a partially applied state), as disclosed in Japanese Patent Laid-Open No. 142970/1990 (i.e., JPA2-142970). The invention, as disclosed in this publication, is constructed, as will be specifically described in the following. When the lock-up clutch is to be changed from the release state to the apply state, it is controlled at first into the slip state to reduce the difference between the RPM at the input side and the RPM at the output side gradually, and the lock-up clutch is then controlled into the complete apply state.
In the system of the prior art, as described above, the running state of the vehicle comes into the lock-up region as the vehicle speed rises thereby to control the application of the lock-up clutch. This control is intended to prevent an abrupt change in the rotation, which might otherwise be caused by applying the lock-up clutch, and the shocks which might accompany the abrupt rotational change. When the lock-up clutch is set in the slip state, however, the abrupt torque fluctuation and the vibrations can be cut while the torque being transmitted at a level. Therefore, the slip control can be effectively executed not only as a transient control for applying the lock-up clutch but also in other situations. In these situations, the slip control may desirably be made together with the control of the state such as the input torque of the prime mover. In the prior art, the slip control is made exclusively on the basis of the difference between the input/output RPMs, i.e., the slip percentage so that it is not always proper. This may deteriorate the shocks or the vibrations.